This pedal can be used to either control volume, or it can be used as an Expression or Control Voltage (CV) pedal - as a modulator, similar to the mod wheel, to control certain parameters of your keyboard. Not all keyboards can use such a pedal, so make sure your keyboard has a jack for CV Pedal, Expression, Control Pedal, or similar.
Some keyboard brands use one type of CV pedal, others use a second type; this pedal is universal, with a switch to select either type. Since the CV pedal adds a new element of expression to your playing, once you try it, you'll never give it up!